Gases that are normally distributed and stored as cryogenic liquids, for instance, atmospheric gases such as nitrogen or compressed natural gas, are sometimes required at pressures exceeding working pressures that can practically be obtained in conventional storage tanks. In such cases a reciprocating pump is used to pressurize a liquid stream removed from the storage tank prior to vaporization of the liquid. Typically, the pump remains idle most of the time but can be started several times during each operating day for periods of up to several hours.
When a cryogenic liquid stream is withdrawn from a storage vessel for purposes of pumping the liquid, the liquid will tend to at first vaporize as the piping connecting the storage tank to the pump and the pump itself are cooled to cryogenic temperature. Additionally, heat is also added to the liquid by the pump due to friction within the moving parts of the pump. If the vaporization occurs before or within the pump, it may prevent priming or cause of loss of prime. Heat added to the fluid to be pumped affects pump performance because of the drop in density. Vaporization causes cavitation, loss of pumping efficiency and accelerated pump wear.
In order to avoid vaporization of the liquid it is known to first subcool the liquid to be pumped. Secondly it is known to provide a supplementary flow in the supply line over and above the pump displacement to clear warm and vaporized liquid that tends to accumulate at the pump inlet regardless of the subcooling available.
In the prior art, subcooling has been done by increasing the static pressure above the body of liquid in the storage tank. For instance, in U.S. Pat. No. 2,850,882, this is accomplished with an external pressure building circuit in which liquid is vaporized and then returned to the head space of the tank so that the liquid pressure is increased. This approach has the distinct disadvantage that the heat added to the tank in the pressure building process is over time transferred to the liquid phase, raising its temperature. As a result, pressure must be continually raised to maintain subcooling until the pressure limit of the storage tank is reached. At that point the storage tank must be blown down to recool the liquid and the tank repressuized. The loss of product in this process is amplified because the temperature of the storage tank metal must also be cycled. In this patent the supplementary flow is established by connecting the pump sump to the gas phase of the tank. This works well when the tank is nearly full since only a small warming of the return liquid is required to establish the circulation flow. However as the tank level is reduced the circulation flow slows until it is insufficient to keep the pump running properly.
U.S. Pat. No. 5,218,827 takes advantage of the normal stratification that is found in storage tanks. Such stratification results in subcooled liquid at the bottom center of the tank and saturated liquid at peripheral regions of the tank. In order to maintain the stratification, subcooled liquid to be pumped is drawn from the center of the tank and from a lower extension of the tank to maintain pressurization of the liquid. Warmed and vaporized liquid is returned from a sump associated with the pump to the periphery of the tank. In this manner stratification is maintained while also maintaining a circulation of liquid to maintain the delivery line from the tank to the pump and the pump itself in a sufficiently cool condition to prevent vaporization of the liquid. This approach represents an improvement over the prior art because the circulation flow is independent of liquid level. The limitation of this invention is that the amount of subcooling established minimal compared to the requirement of the pump.
Another possibility is to subcool the liquid in a known subcooling unit such as disclosed in U.S. Pat. No. 4,716,738. In this patent, liquid to be distributed to use points is split into two streams. It is to be noted that the use points are not pumps. One stream is vented at atmospheric pressure within a phase separator. The venting causes a cool liquid fraction to collect within the phase separator. At the same time, the other of the two streams flows through a coiled heat exchanger submerged within the liquid fraction. The liquid flowing within the heat exchange coil transfers heat to the cooler liquid fraction within the phase separator thereby to subcool the liquid. The resultant subcooled liquid can be dispensed to a variety of use points. The problem with using such a device in connection with a pump is that when the liquid level is at a sufficient height, flow to the vent is cut off. As a result, any inevitable vapor formation or warm liquid at the inlet of the pump will tend to accumulate and thereby cause a decrease in pumping efficiency.
As will be discussed, the present invention provides a method of pumping a liquid from a storage tank and a system for pumping such a liquid in which the liquid can be subcooled to any required level at or near the pump inlet by a subcooling unit that is specifically designed to provide a supplementary flow of liquid to clear any vapor or warm liquid that accumulates near the pump inlet.